Centre of Gravity_01

The COMPOSITE area consists of two distinct regions, square A1 and semi-circle A2. The point 'G' represents the position of the the C.G. of the entire area.

Q1: Why is the LOCUS of the C.G. a straight and not a curve ? When would it be any curve other than a straight line?
Q2 : Predict the position of the C.G. if a similar semi-circle is added to the area on the left side ?